Optical-phonon states of SiC small particles studied by Raman scattering and infrared absorption.

Abstract

The particle-size dependence of frequencies and dampings of optical phonons is studied from analyses of Raman-scattering and infrared-absorption spectra of SiC small particles. The particle size ranges from a few nanometers to more than 1 \ensuremath{\mu}m. The measured spectra are explained qualitatively in terms of a model of a core plasmon and a carrier-free layer on the surface (shell) of the particles, provided that their sizes are larger than about 30 nm. Here we use Mie's scattering equation or the averaged dielectric function of Maxwell-Garnet. The thickness of the shell is 5--40 nm depending on the sample. If the particle size is smaller than about 200 nm, phonon parameters, such as the TO-phonon frequency (${\ensuremath{\omega}}_{T}$), the LO-phonon frequency (${\ensuremath{\omega}}_{L}$) and their dampings, depend on the particle size rather than on the size of the crystallites which constitute the particle. For smaller particles, both ${\ensuremath{\omega}}_{T}$ and ${\ensuremath{\omega}}_{L}$-${\ensuremath{\omega}}_{T}$ become smaller and the damping of the phonon becomes larger. The decrease in ${\ensuremath{\omega}}_{T}$ and increase in the phonon damping are correlated with the lattice imperfection near the surface. The decrease in ${\ensuremath{\omega}}_{L}$-${\ensuremath{\omega}}_{T}$ is related to the absence of a contribution to the macroscopic electric field from the hypothetical atoms outside the particle and to the existence of the surface layer which gives little contribution to the macroscopic field. For particle sizes of a few nanometers, the absorption spectrum exhibits a combination of a crystal-like component and a broad symmetric line as expected for amorphous materials. The origin of the latter is attributed to the ``surface layer,'' which is about 1 nm thick with lattice imperfection.